The present application relates to an optical information recording medium in which information is recorded, erased, or read by changing a state of a recording layer by irradiating the recording layer with light, and by reading the change as a change in an optical response.
A phase-change optical recording medium, which utilizes a transition between a crystal phase and an amorphous phase or between two crystal phases of a crystal 1 and a crystal 2 is known as an optical memory medium that can record, reproduce, and erase information by irradiation of a laser beam. As such a phase-change optical recording medium, for example, a CD-RW (Compact Disc-Rewritable), a DVD-RW (Digital Versatile Disc-Rewritable), and a DVD-RAM (Digital Versatile Disc-Random Access Memory) have been commercially available. Recording materials containing GeSbTe, AgInSbTe, or the like as a main component have been widely known as materials of a recording layer used in the phase-change recording method, and have been practically used in a rewritable optical disc.
Recently, a high-density optical disc compatible with a blue laser wavelength, typically represented by a Blu-ray Disc (registered trademark), and a disc drive device compatible with such a disc have been commercialized. In the standards of such a high-density optical disc such as a Blu-ray Disc, a standard of a double-layer disc which is not included in the standards of a CD-RW, DVD-RW, or a DVD-RAM among rewritable (phase-change) optical discs has been included and practically used. The realization of a double-layer disc in the field of rewritable optical discs has come from the technical finding that a phase-change recording material and a metal thin film used in an information recording layer more readily transmit a blue laser beam than a red laser beam. In addition, desires for a mass storage and a long-time video recording are constantly present for optical information recording media. These desires have also promoted the development of a double-layer disc.
FIG. 15A schematically shows a structure of a double-layer disc (layer structure in the thickness direction of the disc). A phase-change double-layer disc has the following layer structure. Specifically, a first information recording layer 101 is provided on a supporting substrate 100 made of a plastic such as polycarbonate. Furthermore, an interlayer 102 that is transparent to a wavelength of a recording/reproducing laser beam is provided on the first information recording layer 101, and a second information recording layer 103 is provided on the interlayer 102. A light-transmissive protective layer (cover layer) 104 that is transparent to the recording/reproducing wavelength is further provided on the second information recording layer 103. The laser beam used for recording and reproducing is incident from the light-transmissive protective layer 104 side through an objective lens (not shown). The laser beam passing through the objective lens is condensed on the first information recording layer 101 or the second information recording layer 103 to perform recording or reproduction of information.
A feature of the phase-change double-layer disc lies in the second information recording layer 103. The second information recording layer 103 is formed by depositing a dielectric material, a metal, a phase-change recording material, and the like using a sputtering apparatus so as to exhibit a recording/reproducing performance. As shown in FIG. 15A, for example, a typical laminated structure includes, when viewed from the laser beam incident side, a first dielectric portion 111, a recording material portion 112, a second dielectric portion 113, a metal portion 114, and a third dielectric portion 115 in that order. That is, in producing the disc, the layer structure is formed by sequentially laminating the third dielectric portion 115, the metal portion 114, the second dielectric portion 113, the recording material portion 112, and the first dielectric portion 111 on the interlayer 102. Specifically, in producing the disc, the layer structure is formed by sequentially laminating the third dielectric portion 115, the metal portion 114, the second dielectric portion 113, the recording material portion 112, and the first dielectric portion 111 on the interlayer 102. The recording material portion 112 is made of a phase-change recording material. The metal portion 114 functions as a reflective film for a laser beam. The second information recording layer 103 functions as a semi-transmissive information recording layer having a property of transmitting light used for recording and reproducing in the first information recording layer 101. The second information recording layer 103 is designed so that the second information recording layer 103 has a light transmittance in the range of about 45% to 55% and a recording/reproducing power and the reflectivity of the first information recording layer and second information recording layer are equivalent to each other, from the viewpoint of an optical disc recording/reproducing device (drive). As a method of obtaining such a high light transmittance, PCT Publication No. WO03/025922 Pamphlet describes a method in which a high-refractive-index transparent dielectric material is used as a dielectric material in a semi-transmissive information recording layer, the dielectric material being arranged between an interlayer material disposed at a supporting substrate side with respect to the semi-transmissive information recording layer and a metal reflective film in the semi-transmissive information recording layer.
As described above, in the phase-change double-layer disc, it is necessary that the semi-transmissive information recording layer (second information recording layer 103) disposed at the light incident side have a high light transmittance of about 50%. For this purpose, a technique is used in which the thicknesses of the recording material portion 112 and the metal portion 114 (metal reflective film) are extremely reduced. However, because of this structure, it is difficult to ensure reliability of storage in an optical disc media and a recording/reproducing signal characteristic of information. If the light transmittance can be increased without decreasing the thicknesses of the recording material portion 112 and the metal portion 114, the information recording layer can be designed so that the recording material portion 112 and the metal portion 114 have large thicknesses, and thus the reliability and the recording/reproducing signal characteristic can be improved. Consequently, a development of a technique for increasing the light transmittance without extremely decreasing the thicknesses of the recording material portion 112 and the metal portion 114 in a semi-transmissive information recording layer has been desired.
Furthermore, in full-scale operation of digital broadcasting and the like, demands on higher capacity information recording media have been increasing, and an optical disc having a higher storage capacity has also been desired. One method for realizing a higher storage capacity is to further increase the number of layers of an optical disc. For example, a standard Blu-ray single-layer disc has a capacity of 25 gigabyte (GB), and a Blu-ray double-layer disc has a capacity of 50 GB. If the disc can have three layers, the capacity can be increased to 75 GB. If the disc can have four layers, 100 GB can be realized. Furthermore, it is estimated that, by combining a latest signal-processing technique, a capacity of 100 GB can be realized in a three-layer disc, and a capacity of 130 GB can be realized in a four-layer disc. In order to realize a three-layer disc, a third information recording layer is formed at the light-transmissive protective layer side with respect to the second information recording layer, with an interlayer that is transparent to a recording/reproducing laser beam therebetween. Specifically, as schematically shown in FIG. 15B, such a three-layer disc has a layer structure in which a first information recording layer 201, a first interlayer 202, a second information recording layer 203, a second interlayer 204, a third information recording layer 205, and a light-transmissive protective layer 206 are disposed on a supporting substrate 200. The second information recording layer 203 and the third information recording layer 205 function as semi-transmissive information recording layers. As in the second information recording layer 103 shown in FIG. 15A, the information recording layers may have a structure including, when viewed from the laser beam incident side, a first dielectric portion 111, a recording material portion 112, a second dielectric portion 113, a metal portion 114, and a third dielectric portion 115 in that order.
In the case where the third information recording layer 205 having the above structure is formed by further developing the double-layer disc that has been practically used to realize a three-layer disc, it is necessary that the third information recording layer 205 have a transmittance in the range of 65% to 75%. This is a transmittance range obtained by considering the reflectivity of each recording layer of the double-layer disc and the transmittance (about 50%) of the second information recording layer, and considering the balances of the reflectivity from each information recording layer and recording power. In order to increase the reflectivity from each of the first and second information recording layers 201 and 203, a higher transmittance of the third information recording layer 205 is necessary.
A main factor that decreases the transmittance in the information recording layer functioning as the semi-transmissive information recording layer, that is, in the information recording layer other than the first information recording layer 101 or 201 lies in light absorption in the metal portion 114 and the recording material portion 112 made of a phase-change material. Since the metal portion 114 and the recording material portion 112 deeply relate to thermal properties related to recording and reproducing, a rewrite performance, and the like, it is difficult to significantly change the materials of these portions. Therefore, it is assumed that the transmittance is improved by decreasing the thicknesses of the metal portion 114 and the recording material portion 112, but as described above, it is desirable that a significant decrease in the thicknesses be avoided.
As described above, the information recording layer is a laminated film, and is optically a multiple optical interference film. Therefore, by optimizing the thicknesses of the films to be laminated, the information recording layer can be designed so as to have a high transmittance. PCT Publication No. WO03/025922 Pamphlet proposes that in order to increase the transmittance of the second information recording layer 103, a material having a refractive index as high as possible is arranged as the third dielectric portion 115, which is in contact with the interlayer 102 in the second information recording layer 103, though this technique relates to a double-layer disc. The refractive index of the material is 2.4 or more.
However, the light transmittance that can be realized by this structure is less than 65% at most. A further factor is necessary in order to realize a transmittance of 65% to 75% necessary for a three-layer disc. Accordingly, it is desirable to propose a technique for realizing a higher transmittance in a semi-transmissive information recording layer without extremely decreasing the thicknesses of a metal portion and a recording material portion, thereby realizing practical application of a three-layer disc, a four-layer disc, and the like.